Sodium carbonate as inhibitor against corrosion by ammonia-ammonium nitrate solutions



United States Patent 3,011,862 SODIUM CARBONATE AS INHIBITOR AGAINSTCORROSION BY AMMONIA-AMMONIUM NI- TRATE SOLUTIONS Franklin M. Watkins,Flossmoor, 111., assignor to Sinclair Ilsiefining Company, New York,N.Y., a corporation of ame No Drawing. Filed Dec. 21, 1959, Ser. No.860,642 3 Claims. (Cl. 212.5)

This invention relates to a process for preventing corrosion to ferrousmetal containers and more particularly to a process for inhibitingcorrosion to ferrous metal surfaces when contacted with aqueousammonia-ammonium nitrate solutions. Today, large volumes of aqueousammonia-ammonium nitrate solutions are used in agriculture, and thesesolutions are generally corrosive to mild steel surfaces such as thetype which are utilized in the handling and storage of fertilizersolutions and the like. Thus, the industry has necessarily been requiredto utilize the more costly aluminum and stainless steel.

Methods of controlling this corrosion have been devised, but each methodheretofore utilized, has certain shortcomings. U.S. Patent 2,366,796shows that the surface of light steel may be passivated by treating withan electrical potential of 1.22 to 2 volts. This major wealo ness ofthis technique is that the passive film must be reestablished atfrequent intervals since control of corrosion is provided for only aboutdays and if the solution stands for extensive periods of time, theinterior surfaces of the tank begin to corrode. Thus, each tank or carmust be equipped with an electrical system capable of re-establishingpassivity at frequent intervals. This is impractical as most tank andpipe systems are utilized only intermittently as carriers or storagehousings for fertilizer solutions and thus could not be economicallyequipped with the necessary equipment.

An object of this invention is to provide a simple, inexpensive methodof treating ferrous metal containers to prevent corrosion to containerswhich normally occurs during the handling and storage of aqueousammoniaamrnonium nitrate solutions. This invention further pro vides amethod for treating these metallic surfaces so that there will not besubstantial surface corrosion. Normally, inhibiting corrosion of ametallic surface due to one component such as sulfuric acid, etc. wouldnot be satisfactory in preventing corrosion to the same metallic surfaceby another material such as an ammonia solution. Thus, there is noassurance that a surface which has been treated to prevent corrosionunder given conditions will successfully resist corrosion when theseconditions are changed such as by the addition of another corrosiveelement.

This invention provides a method of protecting ferrous metal surfacesagainst corrosion caused by aqueous ammonia-ammonium nitrate solutionswhich include exposing the ferrous metal surface to an aqueous solutionof sodium carbonate for a period long enough to prevent corrosion to thesurface upon subsequent contact with an aqueous ammonia-ammonium nitratesolution. The preferred embodiment of this invention includes a processfor inhibiting corrosion of metallic surfaces utilized in fertilizerhandling systems wherein a minor amount of sodium carbonate, e.g.soda-ash or sal soda is added to water to form a solution. The metallicsurface is usually con- "ice tacted with this solution for a period atleast about 10 hours. There is generally little advantage to be gainedby contacting the metallic surface with the sodium carbonate solutionfor more than 3 to 5 days. The sodium carbonate treatment may beutilized alone to provide corrosion inhibition or further protection maybe afforded by including a corrosion inhibitor in the ammonia solutionwhich inhibitor is effective should the inactivation provided by thesodium carbonate become inadequate. The corrosion inhibitor should becompatible, i.e. not destroy the inactive state of the metal, and may befor instance a minor corrosion inhibiting amount of soluble trivalentarsenic. The arsenic can be added in various forms, such as AS203 in asodium hydroxide solution or complexed with ethylene glycol, in anamount to provide say about 0.1 gram of As O per 100 ml. of the ammoniasolution.

The amount of sodium carbonate in the aqueous treating solution may varydepending upon the peculiar characteristics of the metallic surfaces tobe protected and the existing external conditions affecting corrosion.The amount of sodium carbonate is suflicient to inhibit corrosion and isgenerally at least about .1 percent by Weight of the aqueous treatingsolution, with a preferred amount being from about .5 to 5, 10 or moreweight percent. The maximum amount of sodium carbonate employed is amatter of economics. The metallic containers normally treated with thissolution have ferrous surfaces and may be of the type known as mildsteel surface containers.

This invention is particularly effective in protecting mild steelagainst corrosion by aqueous ammonia solutions such as solutionscontaining a predominant amount of NH.,NO and a minor amount of NHGenerally, the solutions contain about 1 to preferably about 40 to 80%ammonium nitrate and about 5 to 35%, preferably about 10 to 35%, ammoniawith substantially the balance being water, for instance about 10 to 65%It is preferred that the metallic container be treated for a period offrom about 2 to 3 days.

Various tests were conducted under static conditions using mild steelcoupons which were cleaned by surface finishing with a grit surfacesilica carbide abrasive cloth and then contacted for about 3 seconds ata temperature of F. with an aqueous solution having 15% hydrogenchloride. These mild steel coupons were selectively immersed in asolution having 66.8% NH NO 16.6% NH; and 16.6% H 0 and the corrosionrate determined.

EXAMPLE I A mild steel coupon was tested according to the aboveprocedure and found to be corrosive, the rate of corrosion of the couponwas measured as 6255 MDD. MDD is milligrams loss per square decimeter ofsurface area per day.

EXAMPLE II A mild steel coupon was surface finished and treated withhydrogen chloride according to the procedure above and then immersed ina 1 percent by weight solution of sodium hydroxide for one day and thenimmersed in the 66.8% Nl-I NO 16.6% NH;, and 16.6% water solution. Therate of corrosion was 1700 MDD.

Table I illustrates the results obtained by testing a series of mildsteel coupons which were activated by either sand blasting or surfacefinishing, with or without hydrogen chloride treatment, before immersionin a 1% solution of sodium hydroxide.

Table I [Corrosion record of test coupons in a solution of 66.8% NHNO;+16.6% NH +16.6% H20] Coupon 1st Test Retest No. Finish ActivationRate, Deactivation Rate,

MDD MDD 2 Sand blasted 5, 380 1% NaOIEL. Inactive.

d 5,035 One day. Do. 5, 715 doc Do.

5,400 5,355. Inactive.

do 190 do 0.

Surface fin- 5,810 1% NaOH 1,690.

6,255 onedayufl 1,700.

do Inactive.

1 Previous contact with a solution of 66.8% NH NO 16.6% NHQ and 16.6%H2O.

Representative samples of the coupons which were tested and foundsubject to corrosion according to Table I were then immersed in a 1percent by Weight aqueous solution of sodium carbonate for 64 hours andimmersed again in the solution of 66.8% NH NO 16.6% NH and 16.6% water.The results of these tests demonstrated that the coupons which wereinitially corrosive became inactive upon sodium carbonate contact andthat no appreciable corrosion to their surfaces occurred. The results ofthe tests are given in Table II.

4 Table II [64 hours in 1% NBQOOS] Coupon No.

Inactive.

A comparison of Tables I and II demonstrates the ability of sodiumcarbonate solutions to deactivate ferrous metal surfaces consistentlywhile other solutions such as sodium hydroxide are unable to deactivateconsistently similar metal surfaces.

I claim:

1. A procem for inhibiting corrosion to ferrous metal surfaces caused byaqueous ammonia-ammonium nitrate solutions which consists essentially ofcontacting the ferrous metal surface with an aqueous solution of sodiumcarbonate to inhibit corrosion of said ferrous surface caused by aqueousammonia-ammonium nitrate solutions, and contacting the ferrous metalsurface with an aqueous ammonia-ammonium nitrate solution.

2. The process of claim 1 wherein the sodium carbonate solution containsabout 0.5 to 5 percent sodium carbonate.

3. The process of claim 2 wherein the contact of the ferrous surfacewith sodium carbonate solution is for at least about 10 hours and theaqueous ammonia-ammonium nitrate solution contains about 40 to 80%ammonium nitrate and about 10 to 35% ammonia.

References Cited in the file of this patent UNITED STATES PATENTS1,841,825 Kriegsheim Jan. 19, 1932 2,755,166 Marsh July 17, 1956 FOREIGNPATENTS 691,578 Great Britain May 13, 1953 OTHER REFERENCES NACETechnical Committee Reports, Corrosion, vol. 11, No. 4, April 1955, pp.65-67.

1. A PROCESS FOR INHIBITING CORROSION TO FERROUS METAL SURFACES CAUSEDBY AQUEOUS AMMONIA-AMMONIUM NITRATE SOLUTIONS WHICH CONSISTS ESSENTIALLYOF CONTACTING THE FERROUS METAL SURFACE WITH AN AQUEOUS SOLUTION OFSODIUM CARBONATE TO INHIBIT CORROSION OF SAID FERROUS SURFACE CAUSED BYAQUEOUS AMMONIA-AMMONIUM NITRATE SOLUTIONS, AND CONTACTING THE FERROUSMETAL SURFACE WITH AN AQUEOUS AMMONIA-AMMONIUM NITRATE SOLUTION.